1. Field
Exemplary embodiments of the present invention relate to a semiconductor designing technology, and more particularly, to a semiconductor memory device that performs a refresh operation.
2. Description of the Related Art
Semiconductor memory devices such as Double Data Rate Synchronous Dynamic Random Access Memory (DDR SDRAM) devices generally include a plurality of memory banks for storing data. Each of the memory banks also includes tens of millions of memory cells. Each memory cell includes a cell capacitor and a cell transistor, and a semiconductor memory device stores data by charging or discharging the cell capacitors. Theoretically, the amount of charge stored in a cell capacitor is uniform at all times if there is no control. In reality, however, the amount of charge stored in cell capacitors changes due to voltage differences between the memory cells and peripheral circuits. In other words, charges may leak out of charged cell capacitors, or cell capacitors that are discharged may unintentionally become charged. A change in the amount of charge in a cell capacitor signifies that the data stored in the cell capacitor has changed. This means that the stored data may be lost. To prevent the data from being lost, semiconductor memory devices perform refresh operations. Since the refresh operation is a widely known technology, a detailed description of this will not be provided.
As technology advances, the integration degree of semiconductor memory devices increases. The increase in the integration degree of semiconductor memory devices affects the size of memory banks. A decrease in the size of memory banks translates to a decrease in the space between memory cells, which also means that the space between word lines, coupled with the neighboring memory cells, becomes narrower as well. In the past, there has been no problem, or only limited issues, with the gaps between the word lines. However, as the gaps between the word lines becomes narrower and narrower, new problems are arising. Among the newly emerging problems is a coupling effect occurring between word lines.
Word line activation has to be performed to access a given memory cell of a semiconductor memory device. However, as the gap between the word lines becomes narrower, word line activation causes a coupling effect on the neighboring word lines. When the coupling effect occurs in the neighboring word lines, the memory cells coupled with the neighboring word lines may have difficulty retaining the data stored therein and may lose the data.
To overcome the coupling effect problem, the refresh operation cycle is shortened. In other words, the refresh operation is performed at a cycle that is short enough to keep the data in spite of the coupling effect. However, the coupling effect occurs only in some of the word lines, and performing the refresh operation at short intervals in all word lines is inefficient in terms of circuit operation and power consumption. After all, it is not efficient to make the refresh operation cycle of all word lines short since the coupling effect only occurs in a few word lines.